In printing machines, the sheets to be printed are fed by a sheet feeder, due to which the sheets are removed from a stack, over a feeder table furnished with conveyor belts to a sheet set-up or layout location of the printing machine. Trouble-free printing is achieved only when the sheets come to lie on the feeder table before take-over by the grippers exactly at the forward works and at the stop of lateral pulling marks. This sheet set-up is monitored by control devices which are formed of radiation sources and receivers responsive to the radiation (German Democratic Republic Patent No. 97,627). The sheets to be printed are passed under a lamp, and reflect the light emitted by this lamp to a light-sensitive receiver. In this case, the sheet set-up has answered or reported well, and the printing machine continues to operate without interruption. If a sheet, however, does not lie exactly at the forward marks or lateral pulling marks, the light beam is not reflected, the receiver reports "not good" and the sheet travel is interrupted. Likewise, overshooting of the sheets beyond the forward marks is controlled by these control devices.
Furthermore, the sensitivity adjustment of such photoelectric measuring devices with evaluation circuits responsive thereto depends sharply upon the amount of light absorption of the material to be processed. With thin printing materials in which the amount of absorption is very low, the increase of the sensitivity of the photoelectric measurement devices with such responsive evaluation circuits leads to an amplified glare sensitivity which then causes faulty measurements.
What is disadvantageous with these photoelectric measuring devices is that, respectively, a radiation source and a receiver are required, with either the radiation source or the receiver being located in the plane of the feeder table. Furthermore, the non-adjustable sensitivity adjustment of these photoelectric measuring devices have a disadvantageous effect upon the continuity of the printing process.
Briefly restated, the invention provides a system for monitoring the transport of sheets over a feeder table of a rotary sheet-printing machine, which includes the use of photoelectric sensing devices.
In such sheet printing machines, the sheets to be printed are fed by a sheet feeder which takes a sheet at the time from a stack of sheets and moves the sheets to the feeder table, which is equipped with conveyor belts. A perfect print can only be produced when the leading edge of the sheet is positioned on the table exactly aligned with leading edge markers on the table and the sides of the sheets are in alignment with side markers before the gripper grips the sheet, as described hereinabove. The position of the sheet on the table before it is to be gripped is monitored by the sensing devices which include radiation sources combined with corresponding radiation receivers shown generally as the photoelectric sensors 9.
Further still, in photoelectric sensing systems of the prior art which contain associated evaluation circuits, the sensitivity adjustment of the photoelectric sensors are highly dependent upon the light-absorbtion characteristics of the material to be printed. In the case of thin printing materials having very low light absorbtion characteristics, the sensitivity of the photoelectric sensors must be increased and, therefore, they become sensitive to interference from ambient light, which may lead to errors in the sensing process.
Therefore, in the conventional systems using photoelectric sensors based on a light radiation source and a receiver, it is a disadvantage when either one of the latter is disposed in the surface of the feeder table. Also, for the same reason conventional systems, which do not have adjustable photo-sensors, have a disadvantage in maintaining a continuous printing process.
The invention accordingly provides an uncomplicated solution for monitoring the feeding of sheets at any desired location of feeder table.